You can purchase the 15" front setup...for the rear all you need to do is buy an adapter to move your original calipers up, buy either OE 13.8" rear discs and save money. The stock rear calipers on Mustang GT's are the same as the GT500 caliper, and if you buy the OE style bracket to move the caliper you will need to take your rear axles out to put the bracket on.
I have a 14 TrakPak Mustang with 14" Brembos and spent less than $400 to change my stock 11.8" rear discs to 13.8"....kind of waste of money to buy new calipers....IMHO